Zinc cadmium sulfide cathodoluminescent screen

ABSTRACT

An improved viewing screen for use in electron microscopes can be obtained when a phosphor consisting essentially of zinc sulfide and cadmium sulfide, in specific ratios, and a source of an activator material containing specified amounts of an aluminum source and specified amounts of either copper or silver is substrated on a rigid electrically conductive support.

SHEET 1 BF 2 SIXDENIEL FAR|A& DAVID J. HARRIGAN ATTORNEY I N VENTORS.

United States Patent Faria et al.

[ 1 3,655,978 [451 Apr. 11, 1972 [54] ZINC CADMIUM SULFIDE CATHODOLUMINESCENT SCREEN [72] lnventors: Sixdeniel Faria; David J. Harrigan, both of Towanda, Pa.

[ 73] Assignee: Sylvania Electric Products Inc.

[22] Filed: Sept. 4, 1970 [21] Appl. No.: 69,843

[52] US. Cl. ..2 50/80, 250/71 R [51] Int. Cl. .1101] 1/62 [58] Field of Search ..250/80, 71 R; 117/335 C; 25 2/301 .6 S

[56] References Cited UNITED STATES PATENTS 3,197,636 7/1965 Wyattetal. ..250/80 2,623,858 12/1952 Kroger ..252/30l.6S

Primary Examiner-Morton J. Frome Attorney-Norman J. OMalley, Donald R. Castle and William H. McNeil] r [57] ABSTRACT An improved viewing screen for use in electron microscopes can be obtained when a phosphor consisting essentially of zinc sulfide and cadmium sulfide, in specific ratios, and a source of an activator material containing specified amounts of an aluminum source and specified amounts of either copper or silver is substrated on a rigid electrically conductive support.

9 Claims, 4 Drawing Figures PATENTEUAPR '11 m2 3, 655,978 SHEET 2 OF 2 INVENTORS. SIXDENIEL FARIA & DAVID J. HARRIGAN BY QJ ATTORNEY ZINC CADMIUM SULFIDE CATHODOLUMINESCENT SCREEN CROSS REFERENCE TO RELATED APPLICATIONS Co-pending US. Pat. Application ,Ser. No. 7,977 filed Feb. 2, I970, and Ser. No. 8,041, filed Feb. 2, 1970 discloses processes for producing phosphors useful in the present invention. Co-pending US. Pat. Application Ser. No. 7,450 discloses zinc cadmium sulfide phosphors having silver and aluminum as activatorswhich are useful in the present invention. All of the foregoing applications are assigned to the same assignee as the present invention.

BACKGROUND OF THE INVENTION I..Field Of The Invention This invention relates to phosphor screens and morespecifically to fluorescent viewing screens used in electron microscopes or similar devices.

2. Prior Art High voltage electron microscopes are now being used to examine cell structures of biological specimens, and to identify surface defects in metallurgical and solid state physics materials. Electronically and optically magnified specimens are imaged and focused on a fluorescent viewing screen which is generally perpendicular to the electron beam. Magnifications as high as 200,000 times can be achieved with the normal high voltage electron microscopes.

Theoretical limits of the resolving power of the electron microscope as viewed on the fluorescent viewing screen are in the order of a few Angstroms. This high resolution has been achieved by the high .voltage used and the high stability of current passing through'the lens arrangements, including efficient control through the objective and use of pointed filaments insteadof hairpin cathodes.

Although the aforementioned electronic and optical refinements have been helpful in obtaining high resolutions, very little has been done to improve the properties of the viewing screen. As a result, problems still occur, especially at high magnification, when sample thicknessanddensity reduce the penetrations of electrons to a low level, with-resultant loss of image brightness when viewed on the fluorescent viewing screen.

The phosphor normally used in the fluorescent viewing screens is a green,silver-activated zinc cadmium sulfide. If crystals are favorably oriented; i.e., crystals exhibitinggood transmission of electrons, they will allow enoughelectrons to pass through so that the crystals can generally be viewed with ample brightness and clarity of ;theimage-on the screenemploying the aforementioned phosphor. Problems arise, however, if the crystal is not favorably oriented to the electron beam, or if the density and/orthickness isgreat. The electrons are then retarded with resultant loss of image'brightness when viewed on the fluorescent viewing screen. Another problem is normally referred to as beamdamage. Some samplescan suffer heat effectsfrom the high energy electron beam necessary .to produce enough viewingbrightness using the aforementioned viewing screen.

It is believed, therefore, that a fluorescentviewing screen which offers greater brightness, with better viewing resolution and the ability to operate the electron microscope at a lower kilovolt setting to obtain equivalent brightness with no sacrifice in resolution or scanning speed, is anadvancement in the art.

SUMMARY OF THE INVENTION In accordance with one aspect of this invention, there is BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side elevational view of one embodiment of the invention.

FIGS. 2, 3, and 4 are plan views of one embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE PREFERRED EMBODIMENTS For a better understanding of the preferred embodiments of the invention, together with other and further objects, advantages and capabilities thereof, reference is made to the following disclosure and appended claims in connection with the above-described drawings.

Referring now to the invention with greater particularity, it has been found that a viewing screen for use in electron microscopes and similar devices, which comprises a rigid electrically conductive support and a substantially uniform layer of a cathodoluminescent phosphor adhered to the support, wherein the phosphor consists essentially of zinc sulfide and cadmium sulfide in specific ratios, and a co-activator of specified amounts of aluminum and as an activator specified amounts of either copper or silver, has greater brightness, better viewing resolution, and the ability to operate an electron microscope or similar device at lower kilovolt settings with no reduction in resolution or scanning speed.

Blue or green-emitting phosphors can be used, depending on the activators used and weight ratios of zinc and cadmium sulfides. When the silver-activated aluminum co-activator blue-emitting phosphors are desired, the weight ratio of zinc sulfide to cadmium sulfide can vary from about 90:10 to about 97:3, although a weight ratio of from about 92:8 to about 95:5 is preferred. In the green-emitting phosphors, the ratio of zinc to cadmium will vary depending upon the activator material. When the activator is copper and the co-activator aluminum, essentiallykthe same ratios are used as for the blue-emitting phosphor described above, although it is preferred to have ,Zn: Cd ofrfrom about 95:5 to about 90:10. When the activator is silver and the co-activator aluminum, the ratio of ZnzCd can ,vary from about 55:45 to about :35, although ratios offrom A Although various ,methods can be used to achieve adherence-of thephosphor tothe support, depending upon the type of support, one of the preferred methods is to form a suspension of. the phosphor in a suitable transparent solution castable film material and thereafter form a layer of the phosphor and the film materials. Such film materials are well knowninthe art and are discussed in detail in Vol. 9. EN- CYCLQBEDIA OF CHEMICAL TECHNOLOGY, Kirk- Othmerlnterscience Publishers, New York, N. Y.

. phosphor layer is obtained which is generally from about 0.5

provided a viewing screen for use in electron microscopes.

mils to about 10.0 mils thick. Layers greater than about 10 mils can be obtained, but they are generally avoided because of excessive flaking and cracking. Likewise, layers below about 0.5 mils can be obtained, but they are generally avoided to insure flatness of the flatness and evenness of the screen surface.

FIG. 1 is a side elevational view of the viewing screen. The phosphor layer 4 can be adhered on any rigid non-metallic electrically conducting support 2 such as conducting glass, conducting mylar, or metallic electrically conducting supports such as copper, aluminum, stainless steel and the like. Especially preferred, however, are stainless steel and aluminum.

FIGS. 2, 3 and 4 illustrate but three geometric shapes in which the screen can be made. Generally, any geometric shape can be employed; the shape being limited only by the machinability or cutability of the support and the needs of the user. The phosphor 4 can be applied to the entire area of the support 2 or to a section of the support. In practice it has been found to be advantageous to leave a border or section of the support 2 phosphor free to facilitate handling.

To more fully illustrate the present invention, the following detailed examples are given. All parts, percentages, and proportions are by weight unless otherwise given.

EXAMPLE 1 About 10 parts (Zn Cd, )S:Cu,A1 phosphor is added to about 10 parts of a lacquer composition (97.5 percent n-butylacetate 2.5 percent nitrocellulose) in a receptacle containing about 10 parts glass beads (5 mm) and is blended by roll milling for about l to about 15 minutes. The well dispersed phosphor-lacquer composition is poured through a 60 mesh stainless steel screen into a reservoir containing an adjustable steel blade at one end. The reservoir is drawn over a round stainless steel plate at a uniform speed to yield a phosphor screen measuring about mils thick.

The screen made with (Zn Cd )S:Cu,A1 is placed in an electron microscope and compared with a standard fluorescent viewing screen made with (Zn,Cd)S:Ag. The results are tabulated in Table I.

TABLE I Brightness Comparison Standard Screen Screen From K. V. Setting Light Output Example 1 Based on the measurable light emitted from a screen measuring 2 X 4 in. (520 mm), with the meter at a distance of 7 in. (i8 cm.) from the center ot'the screen, in Foot-Lambcrts.

Resolution (at 80 K. V.)

Standard Screen Screen From Example 1 1000 A. Particle Yes Yes I00 A. Particle Yes Yes 20 A. Particle No Yes The above data indicate that there is a substantial increase in brightness and resolution without any loss in scanning speed or residual florescence.

EXAMPLE 2 About l0 parts (Zn Cd )S:Ag,Al phosphor is added to about 10 parts of a lacquer composition (90 percent amyl acetate 10 percent ethyl cellulose) in a receptacle containing about 10 parts glass beads (5 mm) and is blended for about 10 minutes to about minutes by roll-milling. The welldispersed phosphor-lacquer composition is poured through a 60 mesh stainless steel screen into a reservoir containing an adjustable steel blade on one end. The reservoir is drawn over a rectangular aluminum plate at a uniform speed to yield a phosphor screen measuring about 2 mils thick.

The screen made with (Zn ,Cd ,-,)S:Ag,A1 is placed in an electron microscope and compared with a standard fluorescent viewing screen made (Zn,Cd)S:Ag. The results are tabulated in Table ll.

Based on the measurable light emitted from a screen measuring 2 2 X 4 in. (520 mm), with the meter at a distance ot'7 in. l8 cm) from the center of the screen, in Foot-Lamberts.

Resolution (At K. V.

Standard Screen ll 1000 A. Particle Yes Yes 100 A. Particle Yes Yes 20 A. Particle No Yes The above data indicate that there is a substantial increase in brightness and resolution without any substantial loss in scanning speed or residual fluorescence.

While there have been shown and described what are at present considered the preferred embodiments of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

What is claimed is:

1. An electron microscope viewing screen comprising a. a rigid electrically conductive support having a flat smooth face and b. a substantially unifon'n layer of a cathodoluminescent phosphor having a thickness of from about 0.5 mils to about 10 mils and consisting essentially of a host of zinc cadmium sulfide, from about 0.005 percent to about 0.03 percent by weight of an activator selected from the group consisting of silver and copper and from about 0.01 percent to about 0.06 percent by weight of an aluminum coactivator, said phosphor layer adhered to said support and said phosphor exhibiting an emission spectra in the blue to green visible light when subjected to electron bombardment in said electron microscope.

2. A screen according to claim 1, wherein said cathodoluminescent phosphor is suspended in a transparent solution castable film material.

3. A screen according to claim 2, wherein said rigid support is metallic.

4. A screen according to claim 2, wherein said rigid support is non-metallic.

5. A screen according to claim 1, wherein said phosphor has a zinc to cadmium weight ratio of from about :10 to about 97:3.

6. A screen according to claim 5, wherein said activator is silver.

7. A screen according to claim 5, wherein said activator is copper.

8. A screen according to claim 1, wherein said phosphor has a zinc to cadmium weight ratio of from about 55:45 to about 65:35.

9. A screen according to claim 8, wherein said activator is silver.

Pb-ww UNITED STATES PATENT OFFICE CER'HWCATE OF CORREQ'HQN Patent'N o. 3,655,978 Dated April 11, 1972 Inventor) Sixd'eniel Faria and David J. Harrigan It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Col. 2, line 64 "polyvinyl" first instance should read "polyethylene,"

Col. 2, line 74 "flatness" first instance should read "control"- Col. 4, line l5 "22 x 4" should read "2 x 4" Signed and sealed this 12th day of September 1972.

(SEAL) Attestfi EDWARD MQFLEI'CI'IER,JR. ROBERT GOTTSCHALK Commissioner of Patents Attesting Officer 

2. A screen according to claim 1, wherein said cathodoluminescent phosphor is suspended in a transparent solution castable film material.
 3. A screen according to claim 2, wherein said rigid support is metallic.
 4. A screen according to claim 2, wherein said rigid support is non-metallic.
 5. A screen according to claim 1, wherein said phosphor has a zinc to cadmium weight ratio of from about 90:10 to about 97:3.
 6. A screen according to claim 5, wherein said activator is silver.
 7. A screen according to claim 5, wherein said activator is copper.
 8. A screen according to claim 1, wherein said phosphor has a zinc to cadmium weight ratio of from about 55:45 to about 65:35.
 9. A screen according to claim 8, wherein said activator is silver. 